As demands for energy have increased in recent years, oil wells and gas wells are being more widely developed in corrosive environments in which development has not been very frequently done. Line pipe for use in such corrosive environments is required to have a good corrosion resistance.
ISO15156 specifies that the hardness of carbon-steel line pipe that is required to have sulfide stress corrosion cracking resistance (SSC resistance) should be maintained at or below 250 Hv. Further, to provide a certain safety margin, the hardness may sometimes be required to be maintained at a level lower than 250 Hv.
JP 2013-32584 A describes (1) promoting surface decarburization in a quench furnace to reduce the hardness of a surface layer portion, (2) performing quenching in a film boiling condition to reduce cooling rate, thereby reducing the hardness of surface layer portions, and (3) after quenching, grinding a surface layer which is a high-hardness portion.
JP 2002-173710 A describes a method of manufacturing a steel plate having a small added amount of expensive alloying elements and exhibiting good low-temperature toughness and a yield strength not lower than 690 N/mm2.